Electromagnetic shockwave systems for medical purposes, such as lithotripsy, treating pathological tissue conditions and many others, are well known. For example, U.S. Pat. No. 5,233,972 to Rattner describes an electromagnetic or electrodynamic shockwave system in which the shockwaves are generated by a movable element driven by electromagnetic interaction with a stationary element. The shockwave source has a coil arrangement which serves as the stationary element, and which is attached to an insulator member in the shockwave source. A membrane of electrically conductive material is employed as the movable element, and is disposed opposite the stationary coil arrangement. When the coil arrangement is charged with a high-voltage pulse, currents are induced in the membrane in a direction opposite to the direction of the current flowing in the coil arrangement. As a consequence of the opposite magnetic fields arising due to the respective flows of current in the coil arrangement and in the membrane, the membrane is subjected to repelling forces which suddenly and rapidly move the membrane away from the coil.
Ratner describes the transformation of a pressure wave to a shockwave: “A pressure pulse is thereby introduced into an acoustic propagation medium disposed adjacent the membrane. This pressure pulse intensifies during its path through the propagation medium to form a shockwave, as a consequence of the non-linear compression properties of the propagation medium”.
The non-linear effect increases with increased peak pressure. Shockwave formation, therefore, takes place over a formation distance inversely proportional to the wave's peak pressure. The formation distance is shortened by focusing the wave and increasing the peak pressure. Kidney stones disintegration, for example, is commonly carried out by focused waves where shockwave formation starts in water and is completed in (water-equivalent) tissue. A typical shockwave formation distance (from repelling membrane to focal zone) is in the range of 10-30 cm.
Referring to the term “shockwave”, Rattner states: “For simplicity, the waves in the propagation medium will always be referred to herein as shockwaves, and this term will encompass incipient shockwaves in the form of pressure pulses.”
Another example of a prior art electromagnetic shockwave transducer (generator) is found in U.S. Pat. No. 5,230,328 to Buchholtz, et al. Pressure pulses are generated by “driving a membrane with a spiral coil arrangement having terminals connected to a high-voltage pulse generator which charges the coil with high-voltage pulses having an amplitude in the kilovolt range, for example, 20 kV. Such high-voltages can be generated, for example, by capacitor discharges. When the spiral coil arrangement is charged with such a high-voltage pulse, it generates a magnetic field extremely quickly. Simultaneously, a current is induced in the membrane, or at least in the electrically conductive region thereof, which is directed oppositely to the current flowing in the coil. The membrane current consequently produces an opposing magnetic field, causing the membrane to be rapidly moved away from the spiral coil arrangement. The pressure pulse initiated in the acoustic propagation medium, which is preferably a liquid such as water, is introduced in a suitable manner into the subject to be charged with the pressure pulses. As necessary, focusing of the pressure pulses may be undertaken before the pressure pulses reach the subject, for example by means of an acoustic lens.”
U.S. Pat. No. 5,374,236 to Hassler also describes an electromagnetic coil system for generating pressure pulses which are applied to the body surface of the patient by means of a flexible coupling pillow, filled with a liquid medium for acoustic coupling. “As a consequence of the flexibility of the coupling pillow, the spacing of the pressure pulse source from the body surface can be set, while maintaining contact between the coupling pillow and the body surface, so that the focus of the pressure pulses lies in the zone to be treated.”
Shockwaves are also produced and focused by a self-focusing array of piezoelectric elements. Examples include U.S. Pat. No. 4,721,106 to Kurtze et al. and U.S. Pat. No. 5,111,805 to Jaggy et al. Here again the waves propagate through a liquid acoustic propagation medium before reaching the patient.
Focusing shockwaves by a phased array of electromagnetic transducers is described in U.S. Pat. No. 5,131,392 to Jolesz et al. and US patent application 20090275832 to Gelbart et al. Here again the waves propagate through a liquid acoustic propagation medium before reaching the patient.
Thus, prior art electromagnetic and piezoelectric shockwaves transducers are configured to produce pressure waves in a propagation medium, typically water. Sufficient propagation distance enables focusing and shockwaves formation. The transducers and the focusing means are generally circularly symmetric and are configured to produce generally spherical converging waves.